Re: [Freedreno] [RFC PATCH 1/2] drm/msm/dpu: add dsc helper functions

[Abhinav Kumar <quic_abhinavk@quicinc.com>]

On 3/16/2023 9:36 AM, Abhinav Kumar wrote:
> 
> 
> On 3/16/2023 9:23 AM, Dmitry Baryshkov wrote:
>> On 16/03/2023 18:13, Abhinav Kumar wrote:
>>>
>>>
>>> On 3/16/2023 9:03 AM, Dmitry Baryshkov wrote:
>>>> Hi,
>>>>
>>>> [removed previous conversation]
>>>>
>>>>>
>>>>> Hi Dmitry and Abhinav,
>>>>>
>>>>> Just wanted to follow up on this thread. I've gone over the 
>>>>> MSM-specific
>>>>> DSC params for DP and DSI and have found a few shared calculations and
>>>>> variables between both DSI and DP paths:
>>>>>
>>>>> - (as mentioned earlier in the thread) almost all the calculations in
>>>>> dpu_dsc_populate_dsc_config() match dsi_populate_dsc_params() [1]. The
>>>>> only difference in the math I'm seeing is initial_scale_value.
>>>>
>>>> The value in dsi code is valid for initial_offset = 6144. Please use
>>>> the formula from the standard (= sde_dsc_populate_dsc_config) and add
>>>> it to drm_dsc_helper.c
>>>>

Yes, I agree with this part. for rc_model_size we can use 
DSC_RC_MODEL_SIZE_CONST.

initial_offset is already handled in 
https://patchwork.freedesktop.org/patch/525424/?series=114472&rev=2

Then we can use this math:

rc_model_size / (rc_model_size -
initial_offset), keeping in mind that initial_scale_value has three 
fractional bits.

So this would be 8192 / (8192 - 6144) = 4

Then << 3 for 3 fractional bits = 32.

>>>> If I remember correctly the last remaining item in
>>>> dsi_populate_dsc_params() (except mentioned initial_offset) was
>>>> line_buf_depth, see [3]. I'm not sure about setting it to bpc+1.
>>>> According to the standard it should come from a DSC decoder spec,
>>>> which means it should be set by the DSI panel driver or via
>>>> drm_dp_dsc_sink_line_buf_depth() in the case of DP output.
>>>>
>>>>> - dsc_extra_pclk_cycle_cnt and dce_bytes_per_line, which were 
>>>>> introduced
>>>>> in Kuogee's v1 DSC series [2], are used for DSI, DP, and the DPU 
>>>>> timing
>>>>> engine. dsc_extra_pclk_cycle_cnt is calculated based on pclk_per_line
>>>>> (which is calculated differently between DP and DSI), but
>>>>> dce_bytes_per_line is calculated the same way between DP and DSI.
>>>>>
>>>>> To avoid having to duplicate math in 2 different places, I think it
>>>>> would help to have these calculations in some msm_dsc_helper.c 
>>>>> file. Any
>>>>> thoughts on this?
>>>>
>>>> dsc_extra_pclk_cycle_cnt and dce_bytes_per_line are used only in DPU
>>>> code, so they can stay in DPU driver.
>>>>
>>>
>>> They can stay in the dpu driver is fine but where?
>>>
>>> Like Jessica wrote, this is computed and used in 3 places today :
>>>
>>> 1) DSI video engine computation
>>> 2) DP controller computation
>>> 3) timing engine programming
>>
>> Please excuse me if I'm wrong. I checked both vendor techpack and the 
>> Kuogee's patches. I see them being used only in the SDE / DPU driver 
>> code. Could you please point me to the code path that we are discussing?
>>
> 
> DSI code :
> 
> https://gitlab.freedesktop.org/drm/msm/-/blob/msm-next/drivers/gpu/drm/msm/dsi/dsi_host.c#L868 
> 
> 
> DP code:
> 
> Refer to dp_panel_dsc_pclk_param_calc in 
> https://patchwork.freedesktop.org/patch/519837/?series=113240&rev=1
> 
> Timing engine:
> 
> refer to 
> https://patchwork.freedesktop.org/patch/519838/?series=113240&rev=1
> 
> Probably confusion is due to the naming. bytes_per_line is nothing but 
> bytes_per_pkt * pkt_per_line but the concept is common for DP and DSI.
> 
> +        if (phys->comp_type == MSM_DISPLAY_COMPRESSION_DSC) {
> +            phys->dsc_extra_pclk_cycle_cnt = dsc_info->pclk_per_line;
> +            phys->dsc_extra_disp_width = dsc_info->extra_width;
> +            phys->dce_bytes_per_line =
> +                dsc_info->bytes_per_pkt * dsc_info->pkt_per_line;
> 
>>
>>> So either we have a helper in a common location somewhere so that 
>>> these 3 modules can call that helper and use it OR each module 
>>> duplicates the computation code.
>>>
>>> What should be the common location is the discussion here.
>>>
>>> It cannot be dpu_encoder.c as the DSI/DP dont call into the encoder 
>>> methods.
>>>
>>>>>
>>>>> Thanks,
>>>>>
>>>>> Jessica Zhang
>>>>>
>>>>> [1]
>>>>> https://elixir.bootlin.com/linux/v6.3-rc2/source/drivers/gpu/drm/msm/dsi/dsi_host.c#L1756 
>>>>>
>>>>>
>>>>> [2] 
>>>>> https://patchwork.freedesktop.org/patch/519845/?series=113240&rev=1
>>>>
>>>> [3] https://patchwork.freedesktop.org/patch/525441/?series=114472&rev=2
>>>>
>>>>
>>>>
>>